Priority is claimed under 35 USC xc2xa7119(a) based on Japanese Patent Application Serial No. 2001-239112 filed Aug. 7, 2001.
This invention relates to engines typically used for powered outdoor tools and particularly to such engines which are fueled with a gas lubricant mixture.
It is considered desirable to use four-cycle engine technology over two-cycle engine technology, e.g., for powered outdoor hand tools as both noise and emissions are reduced. A typical four-cycle engine is fueled by a vaporized gasoline and air mixture and a gas flow path leads directly from the engine""s carburetor to the engine""s combustion chamber. Such engines provide oil reservoirs that provide the lubricants necessary for lubricating the moving components of the engine. Small engine use typically does not adapt to this form of lubrication. Small engines used for, e.g., portable powered outdoor tools like hedge trimmers and the like are used in a manner where the engine is turned sideways and even upside down during operation and the oil reservoir type of lubrication is not practical.
Accordingly, four-cycle engines have been developed that are fueled by a gas/oil mixture. (See U.S. Pat. No. 4,708,107). The path of the gas-oil flow is arranged so as to flow in and around the moving components and oil from the mixture is deposited on the components to provide the desired lubrication.
Whereas the use of the lubricant bearing fuel provides the desired result, i.e., lubrication of the parts while using four-cycle technology, and thus less noise and emissions pollution, there are problems as compared to prior two-cycle engines.
One problem is in starting the engines, e.g., with a recoil or starter rope (typical for small engine starting). The path of the fuel is substantially extended over a traditional four-cycle engine design and thus the volume of fuel that has to be pumped through the extended passage requires repeated pulls of the starter rope. Further, in the startup mode, because the flow of fuel initially moves slowly through the extended pathway and the lubricant readily collects on the components, following startup and more rapid flow of the fuel, much of the deposited oil re-enters the flow of fuel and the desired ratio of fuel to oil is altered resulting in incomplete combustion. A still further problem addressed by the present engine design is the desire to limit the engine""s speed (revolutions per minute) when the engine is not under load.
The present design reduces the volume of the extended fuel flow passage and thus the fuel that has to be pumped to achieve startup is reduced. The preferred embodiment of the invention provides valve actuating mechanism including a timing gear interconnected to a cam gear from which a cam lifter actuates a push rod and rocker arm, which in combination, controls the engine""s intake and exhaust valves. The arrangement of these components also determines the flow path of the fuel. By strategic use of the periphery of the timing gear and cam gear, the rotation of these gears assists in boosting the fuel flow along the pathway. Also by maintaining a close tolerance around the working components the path is reduced in volume and requires less fuel to fill that volume. Such strategic use of the components and the tightening of the tolerances enables an engine design that provides a volume for the flow path of the fuel that can be matched to the displacement of the piston in a ratio of between two and four-to-one. This has been found to achieve the desired improvement in flow rate to improve both startup and initial idling of the engine without detrimental affect on the thereafter running of the engine.
Overrunning of the engine is also a consideration herein and is controlled at least in part by reducing the size of the fuel intake port entering the combustion chamber, e.g., to a size less than the air intake port entering the carburetor.
The above and further improvements will be more fully appreciated upon reference to the following detailed description and drawings referred to therein.